The mindkiller

 * Original msg to: Se-crawshaw-at-wpg.uwe.ac.uk
 * Carbons sent to: usa-tesla-at-usa-dot-net

Quoting SE-CRAWSHAW-at-wpg.uwe.ac.uk:

> Hi all, its been a very busy Tesla weekend for me, I have
> completed the construction of my latest coil. I was about to 
> fire it up when The Fear struck. I thought I'd better check 
> with you guys that I'm on the right track prior to blowing up 
> my carefully constructed baby. So, coil specs are:

> Input Filter

> Ducati SFMF-6C 6 amp capacitive and inductive filter wired
> backwards i.e. Line = Load

A little light duty perhaps, but it will work fine.

> Variac; 2-4 amps (I think)

A little light. You need heavier. Run for very short periods of
time or you will let the smoke escape.

> Main HV transformer

> 25Kv spark tester
> 50mA (guestimate based on size)
> Laminated exposed core

10 amp draw at 120 volts. 5 amp draw at 240 volts. Check your
variac rating again to make sure your control is matched to your
power supply.

I check these systems before hooking the coil up by setting up a
simple Jacob's Ladder across the HV bushings on the power supply
and spinning the wattmeter for awhile. This simple "static" test 
will get you familiar with the controls and power supply and will
go a long way towards debugging, problem solving, and eliminating
the nervous "heebie-jeebies".

> Should I have the gap across HV bushings ? (Better for Power
> supply ?)

A safety gap is an absolute must have.

> Primary capacitor(s)
> These are across the HV output and consist of 2 caps in series
> to attain working voltage.

> c1 0.0015uF 12Kv  (large ceramic cap, big lugs)
> c2 0.004 uF 20Kv Sangamo mica cap type g4R
> ( I know these are not ideal i.e. low loss but they're all I've
> got). That makes my cap about  0.0015 x 0.004  = 0.00109 uF

> is that a bit small? 

Assuming the rating of the supply transformer is as you state,
your tank circuit capacitance will balance well with .005 micro-
farads capacitance.

> I could add the following caps in parallel, do I need voltage 
> balancing resistors if I do?

> I also have the following caps available

> 4000pf -at-15 Kv unilator ceramic (big)
> 500pF -at-30kV   STC CHV 411p x 2 These are small 1" x 1.5"
> (doorknob caps? suitable for bypass ?)
> Plessey caps from radar set
> 200pF -at-10Kv x 4
> 680pF -at-10kv x 2
> 220pF -at-10Kv X 2
> 330pF -at- 10kv x 2
> 900pf -at-10kvx2

Mix and match city... These caps have a KVDC rating, meaning
you need to run a few in series to get your working voltage.
When placed in series you won't end up with much capacitance.

To be honest; your working voltage is too high and the value of
your capacitance is too low. You would be better off with 12KVAC
input voltage from your power supply and .01 micrfarads of
capacitance in your tank circuit.

> Spark gap
> three series gaps using 1" brass rod and carbon rod (1 gap is
> brass, the other two are carbon rod, should I replace carbon 
> with brass all round ?)

I have never done this. Let us know how it works.
BTW, I edited slightly and removed the wiring descriptions,
which sounded just fine.

> Primary coil

> I have used copper tape for the coil. Again, I know this is not
> ideal but its what I've got and I've used it before with fairly
> good results. The coil is a dish / spiral type

> ID = 6.75 "
> OD = 19.25"
> N= 22 turns
> tape = 20 x 0.1 mm
> 30 degrees rise vertically

> This coil was constructed using four 30 degree triangles of
> drilled circuit board which were then slotted with a bandsaw to
> accept the tape. These boards were then slotted into slots cut
> in the bottom of a plastic bucket and silicone sealed in place.
> The construction is not particularly robust but I think it will
> hold up.

This sounds just fine. I see no problems here.

> Secondary coil

> Total height 23.6"
> Wound Height 18.8"
> wire D 0.4 mm
> No of turns 1150
> 4" diameter
> Perspex (plexiglass) former with perspex endcaps glued on each
> end (hermetically sealed). Lacquered with corona reducing 
> lacquer.

No problems here either.

> Toroid
> stainless steel shotblasted 12" x 3" flat plate in middle

No problems here. This will get you firing, and will work well
with around a kilowatt of input power. 

> RF Ground
> OK I dug a 4' deep hole and buried a 3'long 0.25" copper pipe 
> in there (total depth 7') I then bolted 2 large copper and 
> brass plates and a 2" strip of 1/8" copper plate to the highest
> end of the pipe. About 6" of the strip sticks up above the 
> ground. After I had made the connection I covered it in 
> silicone grease to prevent corrosion. I then made a salt 
> solution using about 1.5 Kg of table salt and poured about a 
> third of it in the hole. As I filled the dirt back into the 
> hole I added more salt solution. Hopefully this will provide an
> adequate ground. We are in a river valley so the water table is
> only about 6' below ground.

This RF ground will be quite adequate for your coil.

> I intend to connect the tesla coil RF ground (spark gap shield)
> to this by means of four lengths of the copper tape used in the
> primary coil, again I've got to use what I've got.

No problems at all.

> Safety / EMI

> There is no filtering of the HV circuit yet. I have some 0.5 "
> ferrite rod which I am going to wind 20 turns of magnet wire 
> onto and have one of these in seies with each HV line. Bypass 
> caps I can use some of the above across the HV bus, Which ones,
> any idea? 

Try about 500 pf at 60 kvdc.

> With my type of transformer should I ground the core to the RF
> ground ?

If for nothing else but safety's sake, I would.

The rest of your project looks good. You should be able to fudge
a bit and get some spark pretty quickly, but you need to work
on your power supply and HV tank circuit capacitor situation.
The coils, gaps, and tank circuit bussing sound A-OK. The variac
needs to be upgraded if your ratings are correct. The step-up
transformer is running too high a voltage to get started: this is
higher than I run on my big coil. You might try getting a 2:1
step-down transformer and placing this on the primary of your
step-up. This will bring your working tank circuit voltage to
around 12KVAC which should open up your capacitor selection. The
2:1 step-down will also act as an isolation transformer which
should really cut your RF leakage into the 50/60 Hz supply lines. 

I hope these suggestions help.

Richard Quick

... If all else fails... Throw another megavolt across it!
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